This ELU question motivated the following, but I abbreviate all clauses. Also, lest 'then' be written twice in the same sequence of events, interpret the adverb 'then' to precede the adverb 'thence'.

Unless is a kind of negative of if—think of [...] unless as very much like “if . . . not.” [...]

[1.] Unless,  if A, [then] B,  [thence] C.

[2.] In other words,   if it is the case that  if A, then B,   then[ce] C;
otherwise, C. There are two conditionals operating here.

In general,  unless X, Y  =  X, if not Y.  =  [3.]  If not Y, then X. 

In [1.] above,   X  =  if A, [then] B.    And  Y = C.

Then using [3.], [1.] becomes:

[4.]  If not C, then  if A, [then] B.

But [4.] differs from [2.]; so which is wrong?
Please explain and show all steps and thought processes.

  • Why do you think 4 differs from 2? – Keelan Jun 2 '15 at 4:24

There are three standard ways of describing the logical form of "A unless B" where A and B are sentences:

a. A or B
b. If not-A then B
c. If not-B then A

These three are, happily, logically equivalent.

In your description, item [2] uses one form (my b) and item [4] uses another form (my c).

  • unless you see the very ugly person in front of the car, floor the accelerator. floor the accelerator or see the very ugly person in front of the car. hey, If not floor the accelerator, then see the very ugly person in front of the car. – Cheers and hth. - Alf Jun 2 '15 at 4:25

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